SEOUL, South Korea — North Korea fired a suspected long-range ballistic missile into the sea Monday in a resumption of its weapons testing activities, its neighbors said, as the North vows retaliatory steps against U.S. and South Korean moves to boost their nuclear deterrence plans.

South Korea’s military said in a statement that North Korea launched what appeared to be a long-range ballistic missile from its capital region Monday morning. It said South Korea maintain a readiness while exchanging information about the launch with U.S. and Japanese authorities.

Japan’s Defense Ministry said it also spotted a ballistic missile launch by North Korea. A ministry statement said that the missile was still in flight and that it was expected to land in waters outside the Japanese Exclusive Economic Zone.

Japanese Prime Minister Fumio Kishida told reporters that he had so far received no report of injuries or damages from the missile launch and that he planned to hold a National Security Council meeting to discuss the test.

The launch came hours after South Korea reported North Korea conducted a short-range ballistic missile test into the sea Sunday night. It was the North’s first weapons launch in about a month.

Observers said the North’s back-to-back launches were likely a protest against the moves by South Korea and the United States to bolster their nuclear deterrence plans in the face of North Korea’s evolving nuclear threats.

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Senior U.S. and South Korean officials met in Washington over the weekend and agreed to update their nuclear deterrence and contingency strategies and incorporate nuclear operation scenarios in their combined military exercises next summer.

On Sunday, North Korea’s Defense Ministry slammed its rivals’ move to include nuclear operation scenarios in their joint drills, describing it as an open threat to potentially use nuclear weapons against the North. It vowed to prepare unspecified “offensive countermeasures.”

“The armed forces of (North Korea) will thoroughly neutralize the U.S. and its vassal forces’ attempt to ignite a nuclear war,” the North Korean statement said. “Any attempt by the hostile forces to use armed force against (North Korea) will face a preemptive and deadly counteraction.”

Animosities between the two Koreas deepened after North Korea launched its first military reconnaissance satellite into space Nov. 21 in violation of U.N. bans.

South Korea, the U.S. and Japan strongly condemned the launch, viewing it as an attempt by the North to improve its missile technology as well as establish a space-based surveillance system.

South Korea announced plans to resume front-line aerial surveillance in response. North Korea quickly retaliated by restoring border guard posts, officials in Seoul said. Both steps would breach a 2018 inter-Korean deal on easing front-line military tensions.